Novel polyimides and compositions thereof that are soluble in phenolic solvents

ABSTRACT

A POLYIMIDE HAVING A RECURRING STRUCTURAL UNIT OF THE FORMULA   -(CH2-CH2-CH2-(2,4,8,10-TETRAOXASPIRO(5.5)UNDEC-3,9-YLENE)   -CH2-CH2-CH2-(1,3-DI(O=)ISOINDOLIN-2,6-YLENE)-CO-   (1,3-DI(O=)ISOINDOLIN-6,2-YLENE)-   CAN BE PRODUCED BY CAUSING A 3,3&#39;&#39;484&#39;&#39;-BENZOPHENONETETRACARBOXYLIC ACID COMPOUND AND 3,9-BIS(3-AMINOPROPYL)-2,4,8,10-TETRAOXASPIRO(5,5)UNDECANE TO CONTACT EACH OTHER UNDER CONDITIONS CONDUCIVE TO CONDENSATION, SUCH A POLYIMIDE IS BLENDED WITH A CONVERTIBLE POLYSTER AND A PHENOLIC SOLVENT TO PROCUCE A COMPOSITION WHICH IS SUITABLE FOR USE AS AN ELECTRICALLY INSULATING VARNISH FORMING A COATING FILM OF HIGHLY DESIRABLE PROPERTIES.

Jan. 11, 1972 MUNEHlKO SUZUKl ETAL 3,634,304

NOVEL BOLYIMIDES AND COMPOSITIONS THEREOF THAT ARE SOLUBLE IN PHENOLIC, SCLVENTS Filed May 2l, 1969 INVENTORS Mana-afm Suzuki ETSMQ HOQt-Auln mit. WAR.

ATTORNEYS United States Patent O 3,634,304 NOVEL POLYIMIDES AND COMPOSITIONS THEREOF THAT ARE SOLUBLE 1N PHE- NOLIC SOLVENTS Munehiko Suzuki, Yokosuka-shi, Etsuo Hosokawa, Yoko.

hama-shi, and Misao Waki, Kawasaki-shi, Japan, assignors to Showa Densen Denran Kabnshiki Kaisha, also known as Showa Electric Wire and Cable Co., Ltd., Kanagawa-ken, Japan 3,634,34 Patented Jan. 1l, 1972 ICE ing varnishes, each prepared by causing an aromatic tletracarboxylic acid dianhydride and an aromatic diamine to react in an organic polar solvent at a temperature below 50 degrees C. and obtaining a polyamide acid solution, have been known.

When such a conventional polyimide resin insulating varnish is applied onto an electroconductive material such as copper wire or an aluminum wire and baked, the polyamide acid undergoes dehydrocyclization and Filed May 21, 1969, Ser. No. 826,491 converts into a polyimide, thereby forming an insulating Int. Cl. C08g 20/32, 41/04, 51/34 varnish film of high heat resistance.

U'S' Cl260-33'4P 4 Claims While these known polyimide resin insulating varnishes have excellent properties once they are applied and baked, they are accompanied by certain diiculties.

ABSTRACT 0F THE DISCLOSURE 15 One difficulty is the high price of the varnish due to the A polyimide having a recurring structural unit of the Solubility of the polyamide acid constituting the preformula dominant constitutent thereof in only expensive organic 0 o l t o-o o Ha CH2 IC CH2CH2CH2CH o CHCHZCHZCHZ-N (i, N

\0-Cz \CHz-O/ C C o t can be produced by causing a 3,3,4,4'-benzophenonepolar solvents. Another diiiiculty is the extreme instabiltetracarboxylic acid compound and 3,9-bis(3aminopro 30 ity of the polyamide acid, which readily converts into pyl)-2,4,8,10tetraoxaspiro[5,5]undecane to contact each an insoluble polyimide even at room temperature, giving other under conditions conducive to condensation. Such rise to gelation of the varnish, whereby the varnish must a polyimide is blended with a convertible polyester and be stored under refrigeration. Such dii'iiculties are ina phenolic solvent to produce a composition which is dustrially disadvantageous features of these known insuitable for use as an electrically insulating varnish form- 3,. sulating varnishes.

ing a coating lm of highly desirable properties. o Accordingly, the reaction for producing such a'polyamide acid has heretofore been carried out at loW temperatures, at which the tetracarboxylic acid does not re- BACKGROUND 0F THE INVENTION act with the diamine unless it is in the state of a dian- 40 hydride. For this reason, it is disadvantageously necessary This invention relates generally to polyimides and to heat the tetracarboxylic acid dianhydride for a long compositions thereof and more particularly to new polytime prior to carrying out the reaction thereby to drive imides derived from 3,3',4,4'benzophenonetetracar ofi thoroughly the adsorbed water.

boxylic acid and 3,9-bis(l-aminopropyl)-2,4,8,10-tetra- Inventions relating to polyimides obtained by heating oxaspiro[5,5]undecane and new compositions produced V and causing reaction of an aromatic tetracarboxylic by dissolving these imides and convertible polyesters in 4 acid and an aliphatic diamine are disclosed in U.S. Pat. phenolic solvents. 2,710,853 (patented June 14, 1955, Polyimides of Pyro- Furthermore, this invention in still another aspect theremellitic Acid) and U.S. Pat. 2,731,447 (patented Jan. of relates to improvements in electrically insulating 17, 1956, Novel Polyimides). The polyimides of these varnishes and more speciiically to new insulating varinventions, however, have very low solubility of organic nishes of polyimide resins which have high stability and solvents and cannot be used in practice as constituents are capable of forming coating lms having, simultaof insulating varnishes.

neously, excellent heat resistance and mechanical prop- Furthermore, baked varnish iilms obtained through the erties, and for which, moreover, inexpensive phenolic use of the above described known insulating varnishes solvents can be used as varnish solvents. of polyimide resins are deficient in various mechanical In recent years continual efforts have been directed properties beginning with wear resistance, whereby when toward miniaturization of size, elevation of performance, a magnet wire on which any of these varnishes has been and expansion of iields of applications of electrical applied and baked is wound into a coil by an automatic machines and equipment, such as motors and generators, winding operation, there is a tendency for the varnish in which coils are used. In accordance with this trend, iilm to be damaged.

there has grown a demand for development of So-called magnet wires which can be used even at high temperatures.

As an insulating varnish for a heat-resistant enameled wire of this character, so-called polyimide resin insulat- Heretofore, the technique of improving the various properties of an insulating varnish `film by admixing one or more resins amply possessing the desired properties with the insulating paint has been known. However, known polyimide resin insulating varnishes as described 3 above have no mutual solubility whatsoever with resins such as convertible polyesters which have excellent mechanical properties. Consequently, it has not been possible to resort to the above mentioned technique of improving varnish film properties in the case of the above described known polyimide resin varnishes.

In view of the above described state 0f the art, we have studied the above described problems. As a result we have discovered that polyimides derived from a certain tetracarboxylic acid and a certain diamine are readily soluble in inexpensive phenolic solvents and that, moreover, convertible polyesters can be dissolved in large quantities in the resulting polyimide solutions, whereby it is possible to produce an insulating varnish capable of forming an insulating varnish film which possesses, simultaneously, heat resistance and excellent mechanical properties.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide polyimides which are soluble in phenolic solvents.

Another object of the invention is to provide compositions each comprising a polyimide of the above stated character, a convertible polyester, and a phenolic solvent.

A further object of the invention is to provide insulating -varnishes of new polyimide resins each having, as principal constituents thereof, a polyimide and at least one polyester.

According to the present invention in one aspect thereof, briey summarized, there is provided polyimides each having a recurring structural unit of the formula According to the present invention there is further provided a method for producing polyimides of the above stated character.

According to the present invention, in still another aspect thereof, there are provided compositions each comprising as a blend a polyimide of the above stated character, a convertible polyester, and a phenolic solvent. One application of these compositions is their use as electrically insulating varnishes.

The nature, details, and utility of the invention will be more clearly apparent from the following detailed description beginning with general considerations and concluding with specific examples of practice constituting preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing, the single ligure is an infrared analysis chart of a polyimide suitable for use according to the invention.

DETAILED DESCRIPTION A polyimide according to the invention can be produced, in general, by: blending 3,3', 4,4benzophenonetet racarboxylic acid (hereinafter designated by the abbreviation BTA), a functional derivative thereof as, for example, a dianhydride of BTA(hereinafter designated by the abbreviation BTDA), or a lower alkylester of BTA, and approximately thc same mol quantity of 3,9-bis(3 4 aminopropyl) 2,4,8,l0 tetraoxaspiro [5,5] undecane of the formula H2N-CH2'CH2.

(hereinafter designated by the abbreviation BTU) together with a phenolic solvent in proportions such that the sum of the quantities of the two first-named components (i.e., BTA or a functional derivative thereof or a lower alkylester of BTA and BTU) is from 5 to 60 percent by weight of the total sum of the entire batch thus blended; placing the batch in a suitable reaction vessel; and agitating the batch for a period of from 1 to 6 hours at a temperature above 8() degrees C.

The mol ratio of the above mentioned two components need not strictly be equal. Rather, a blend in which the quantity of one reaction component is somewhat in excess of that of the other component is even more preferable.

However, if the dierence between these quantities is excessive, the degree of polymcrisation of the polyimide produced will be restricted with the result that the characteristics of the baked coating lm of the insulating varnish will be impaired. Accordingly, this difference is preferably held within l0 mol percent of the smaller quantity.

We have found further that when the quantity of the reaction solvent is excessively large relative to that of the reactants, the reaction does not progress thoroughly. On the other hand, when the quantity of the solvent is deciently small, the concentration of the resulting polyimide solution becomes excessively high, which result is undesirable since it becomes difcult for the convertible polyester added in the succeeding process step to dissolve.

In this reaction, furthermore, the reaction temperature has a great influence on the progress of the reaction.

More specifically, this polyimide-forming reaction may be considered to take place in accordance, for example, with the following dehydration reactions of two steps (dehydration and dealcoholisation reactions in the case wherein a lower alkylester is used).

HOOG COOH IIOOC Where x represents and Y represents -CH2'CH2'CH2- We have found that when the reaction temperature is below 80 degrees C., the reaction indicated by Formula Il progresses with difficulty, and the reaction solution becomes one in which the proportion of the polyamide acid is high. As a result, the mutual solubility with the convertible polyester added in the succeeding process step is lowered, and, in addition, blisters or pin holes tend to be formed in the coating film since dehydration occurs at the time of baking of the varnish film.

Furthermore, since the reaction of Formula I is also a dehydration reaction, the reaction system will reach an equilibrium state if it is difficult for the separated water -to escape out of the system, whereby it will be difficult to obtain a polyimide of high molecular weight.

Accordingly, it is preferable that the above described reaction be carried out at a temperature above 100 degrees C. and below the boiling point (from 200 to 220 degrees C.) of the solvent.

When the reaction is carried out under reduced pressure a reaction temperature which is lower than that in the case of reaction at atmospheric pressure can be used. Furthermore, the reaction temperature can `be temporarily lowered below 80 degrees C. during the reaction process without any adverse effect.

A polyimide produced in the above described manner is characterised by an infrared analysis chart as Ishown in the accompanying illustration and readily dissolves in a wide range of proportions in phenolic solvents. lExamples of such phenolic solvents are phenol, xylenol, and cresol. Poor-solvents such as naphtha, toluene, and xylene can also be used when the additive quantities thereof are below 60 percent.

The polyimide thus produced, in general, is substantially totally soluble (at 30 degrees C.) in the phenolic solvent and, in general, has a relative viscosity (0.5 g. polymer/ 100 ml. m-cresol, 30 degrees C.) higher than 0.1. The m-cresol used for this relative viscosity and the examples of practice to follow was of the following composition.

Percent Phenol 3.2 o-Cresol 3.3 p-Cresol 39.6 m-Cresol 53.9

A polyimide produced in the above described manner, in the form of a solution, can be used directly, as it is, as an insulating varnish capable of forming a heat-resistant coating film. However, the baked coating film resulting from this solution is not always completely satisfactory in mechanical properties.

In accordance with the present invention, for the purpose of improving these mechanical properties of the baked coating film, a known convertible polyester or a solution of a convertible polyester in an organic solvent, that is, a polyester insulating varnish, is further added to the above mentioned polyimide solution, and the resulting combination is uniformly mixed. At this time, a diisocyanate or isocyanate generator may, of course, be added simultaneously together with the convertible polyester.

The convertible polyester added in this manner is mutually dissolved with the polyimide solution in almost any ratio to impart a substantial improvement in the properties of the product over those of the polyester insulating varnish by itself or those of the polyimide solution by itself even when, with respect to parts by weight of the resin content within the polyimide solution, a small quantity of the order of 10 parts by weight of the convertible polyester is added or, conversely, when a large quantity thereof of the order of 1,000 parts by weight is added.

The quantity of the diisocyanate or isocyanate generator to be added is preferably less than 50 parts by weight with respect to 100 parts by weight of the resin content of the insulating varnish (polyimide solution plus convertible polyester).

The term convertible polyester is herein used to designate a so-called primary condensate which is soluble in a phenolic solvent and has heretofore been used as the polyester component of polyester insulating varnishes, and which is a polyester capable of undergoing further condensation reaction under the action of heat and (or) a catalyst to be converted into a cross-linked structure. This convertible polyester can be prepared, in general, by heating and causing reaction of a mixture of suitable proportions of a dicarboxylic acid, a divalent alcohol, and an alcohol or an acid of a valency of three thereabove as a cross-linking component in a phenolic solvent.

Therefore, this polyester is one which has a hydroxyl group and a carboxyl group which are capable of reacting further to form a three-dimensional network and are in a free state or a state wherein the reactivity thereof is blocked because of etherification or esterification with a lower alcohol.

Examples of dicarboxylic acids suitable for use as one of the above enumerated starting materials are terephthalic acid, isophthalic acid, adipic acid, pimelic acid, and suberic acid. In addition, lower alkylesters of these acids can be similarly used. Examples of suitable divalent alcohols are ethylene glycol, diethylene glycol, trimethylene glycol, tetramethylene glycol, butanediol, and 2,2dimethyl-1,3propanediol. Examples of trivalent or higher valency alcohols or acids are glycerine, trimethylolpropane, pentaerythritol, tris (hydroxymethyl) isocyanurate, diglycerol, cyanuric acid, isocyanuric acid, and trimellitic acid. Lower alkylesters of these acids can be similarly used.

Among the various possible combinations of the above described starting materials, one suitable combination we have found comprises approximately equal quantities of terephthalic acid, ethylene glycol, and glycerine. We have found further that substitution of a portion of this combination with another acid or alcohol also produces desirable results.

Furthermore, the convertible polyester in the present invention includes those of modified as, for example, a polyester prepared by further adding a small quantity of a reactant such as an unsaturated fatty acid to the starting materials, as set forth in the specification of British Pat. 1,088,323, or a convertible polyester already produced to which a modifier such as a diisocyanate has been added.

A convertible polyester suitable for use according to the invention can be produced as illustrated by the following specific example of procedure. The above described starting materials, together with a phenolic solvent in a quantity of the order of 20 percent by weight of the total weight thereof, are placed in a suitable reaction Vessel and heated from room temperature to a maximum temperature of from 230 to 250 degrees C. at a temperature rise rate corresponding to a time of from 6 to 9 hours for this temperature rise thereby to produce the convertible polyester as a reaction product.

While this reaction is completed after the reactants have been heated to a final temperature of approximately 230 to 250 degrees C., the molecular weight of the reaction product thus produced can be further increased by maintaining the product at the `linal temperature for a period of from 1 to 4 hours. It is necessary, however. to cause the reaction to stop prior to the start of gelation of the resin formed in carrying out this procedure of holding the reaction product at the iinal temperature. For this purpose, a phenolic solvent is poured onto the heated convertible polyester. The convertible polyester obtained in this manner is then filtered to remove foreign matter and then used.

Convertible polyesters of the above described character suitable for use accordingY to the invention include polyester resin solutions, i.e., polyester insulating varnishes sold on the market under trademark on product names such as: Alkanex (General Electric Company, U.S.A.), Tcrebec (Farbenfabriken Bayer AG, Germany), each of which is a polyester insulating varnish synthesised from dimethyl terephthalate, ethylene glycol, and glycerinc; Isonel (Schenectady Varnish Company, U.S.A.), which is a polyester insulating varnish synthesised from terephthalic acid, ethylene glycol, glycerine, and tris (hydroxyethyl) iSO- cyanurate; and Desmophen (Farbenfabriken Bayer AG, Germany).

Examples of diisocyanate or isocyanate generators suitable for adding to the insulating varnishes of the invention are diphenylmethane-4-4'-diisocyanate, diphenylether- 4.4diisocyanate, diphenylsulphide-4.4'-diisocyanate, diphenylsulphone 3.3'diisocyanate, diphenylbenzene-4-4- diisocyanate, tolylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, phenolic stabilisers of these diisocyanates, and isocyanates sold on the market under product names such as Desmodur AP Stable, Desmodur CT Stable, Desmodur T, Desmodur N, Desmodur TH, and Desmodur R (Farbenfabriken Bayer AG, Germany).

The preparation of the polyimides for use in accordance with the invention will be disclosed hereinafter with respect to a specific example of method. As mentioned hereinbefore, while the BTA itself can lbe used in its acid form, an anhydride or a lower alkylester thereof can be similarly used,

Accordingly, not only BTA but also BTDA and tetraalkylester of BTA will each be considered to be identical to a di-alkylester of BTA.

The acid values and other characteristics of BTA and anhydrides and lower alkylesters of BTA are set forth in Table l.

In order to indicate still more fully the nature and utility of the invention, the following specific examples of practice constituting preferred embodiments thereof and results are set forth, it being understood that these examples are presented as illustrative only and that they are not intended to limit the scope of the invention.

Example 1 322.2 grammes (g.) of BTDA, 274.4 g. of BTU, and 560 g. of xylenol were placed in a three-neck flask provided with a thermometer, a condenser, and an agitator. The temperature within the flask was then raised to 150 degrees C. as the reactants were agitated and was held at this value for two hours while the agitation was continued, the reaction being stopped when generation of water vapour completely stopped, whereupon a polyimide was obtained. The quantity of water distilled during the reaction was 35 ml.

Examples 2 through 14 In a manner similar to that set forth in Example l, polyimides were prepared under the blending and reaction conditions set forth in Table 2.

TABLE 2 Acid com- Reaction ponent/ conditions, Pressure of Acid Aeid B.t.u. (mol Reaction solvent temp. C.) reaction Example No. component value ratio) (quantity blended) time (hr.) system 696. 9 1/1 Xylenol (60%) 150 2 Atmospheric. 620 1/1.1 Metacresol (80%). 120)(2 10 mm. Hg. 332 1/1 Phenol (40%) 130X2 100 mm. Hg. 462 1.1/1 170)(2 20 mm. Hg. 287 200 1 Atmospheric. 135 180X2 20 mm. Hg. 0 180)(2 20 mm. Hg. 4.24 180X2 20 mm. Hg. 210 180)(2 20 mm. Hg. 120 1/1 d 180)(2 20 mm. Hg. U 1.1/1 d0 180 2 20 mm. Hg. 620 1/1 Xylolrzliyhtha 150X1. 5 150 mm. Hg.

U 696. 9 1.1/1 Xylenol/Xylene SOXI :9/1 (60%) 150 2}Atmospherie. 696.9 l/1.1 Xylenol/toluene 80 6 100 mm. Hg.

alkylesters of BTA can be isolated with purities above 99 percent, but it is difficult, strictly speaking, to isolate the monoanhydride and mono, di, and tri-alkylesters of BTA. Therefore, these compounds will be identified by their acid values or degrees of esterication in the following examples.

Accordingly, a mixture of l mol of BTA and 1 mol of BTDA will be considered to be identical to BTA monoanhydride. Furthermore, a mixture of l mol of BTA and l mol of a tetra-alkylester of BTA, a mixture of l mol of a mono-alkylester of BTA and 1 mol of a trialkylester of BTA, and a mixture of 1 mol of BTA, 1 mol of a monoalkylester of BTA, l mol of a di-alkylester of BTA, l mol of a tri-alkylester of BTA, and l mol of a tetra- Each of the polyimide solutions obtained in the above described manner was dropped into methanol to cause precipitation, and the precipitate was washed several times with methanol and then dried under a vacuum at room temperature, whereupon a polyimide in the form of lightyellow powder was obtained.

Infrared analysis (KBr tablet method) was carried out with respect to each of these polyimides, whereupon a chart as indicated in the accompanying illustration was obtained, and absorption of 1,715 cm.1 and 1,775 cml, which is a characteristic of imides, was conspicuously observable. Ultimate analysis was also carried out with results as indicated below with respect to one example,

whereby it was verified that the products were the desired polyimides.

Measured Calculated value value 1 Element) (percent) percent l? 64. 43 64. 28 `H 4. 76 5. 04 )L 4. 74 5. 00

The principal properties of this polyimide were as follows.

Softening point: 195 to 210 C. Heating loss (200 C. 1 hr.): 4.2% Solubility (polyimide/solvent: 10/ 90) Examples 16 through 33 By the procedure set forth in Example other resin solutions respectively of the blends and particulars as shown in Table 3 were produced.

TABLE 3 (A) (B) `(C) (D) (E) (F) Polylmide Non-volatile solution (A)/(B)/(C) content (per- Viscosity (Example Convertible polyester Dilsocyanate or (wt. ratio cent) (200 (poise) Example No. No.) or polyester varnish isocyanate generator of solids) C XL hr.) (at C.)

l Desmophen F 950.. 100/100/0 40. 0 38. 2 2 100/10/0 4l. 2 41. 5 3 100/1000/0 40. 2 37. 4 4 100/20/0 39. 9 40. 4 5 Terebee F 100/900/0 42.9 39. 5 6 Desmophen 800 Diphenylmethane-4.4 100/400/ 10 38. 7 36.3

dllsocyanate. 7 Desmophen 900 Dlphenyt'lether-iAdliso- 100/400/1 41.8 38. 5

eyana e. 8 Desmophen 1000 Dlphenylsulphlde-4.4 100/400/250 39. 5 37. 2

diisoeyanate. 9 Desmophen 1100 Diphenylsulphone-4A- 100/400/20 40.7 40. 7

dllsocyanate. 10 Desmophen 1200 Diphenylbenzene-4-4 100/100/10 40. 5 38. 6

diisocyanate. l1 Desmophen 2000 Tolylene disocyanate 100/100/10 40. 1 38. 1 l2 Desmophen 2100 Hexamethylene diiso- 100/200/5 39.3 36.9

eyanate. 13 Desmophen 2200- Octamehylene diiso- 100/200/3 39.6 36. 0

eyana e. 14 Desmophen VL 100. Desmodur AP, stable. 100/300/5 40. 9 39. 6 16 Alkanex 9504 Desmodur CT, stable. 100/400/10 38. 8 34. 5 16 Isonel 200 Desmodur P..." 100/400/5 41. 4 37. 0 17 Terebec F. 100/50/0 40. 7 41. 0 18 Isonel 200- 100/900/0 40. 4 39. 2 19 Alkanex 9504. 100/100/0 38.6 35. 7

Completely soluble: phenolic solvent, N-methyl-2- pyrrolidone Partially soluble: cyclohexanone Totally insoluble: toluene, methylethyl ketone, meth- `snol, ethyl Cellosolve, acetone Inherent viscosity (0.5% in metacresol at 30 C): 0.73 Press formability (200 C., 10 sec., 200 atm.): becomes a brittle, glass-like structure.

A solution of Desmophen F 950 in xylenol was added to the polyimide solution prepared by the procedure of Example l in a quantity such that 100 parts of the resin content of the Desmophen solution was added with respect to 100 parts of the resin content of the polyimide The resin solutions thus produced were applied as coating on annealed copper wire of 1.0-mm. diameter similarly as in Example l5 and baked, whereby respective enameled wires were obtained. The insulating film thicknesses of these enameled wires were all within the range of from 0.046 to 0.047 mm. The characteristics of each of these enameled wires are indicated in Table 4.

For the purpose of comparison, corresponding characteristics of enameled wires, designated as Reference Examples Ref. l through Ref. 4, inclusive, are also shown in Table 4, these reference enameled wires being produced by applying as coating polyester insulating varnish and polyimide resin varnish independently on annealed copper wire of 1.0-mm. diameter and baking the applied varnish under the same conditions as in the other examples.

Alkanex 9504 Was used in Reference Example Ref. l, Terebec F in Ref. 2, and Isonel 200 in Ref. 3, and in Ref. 4 use was made of a polyimide resin insulating varnish (a solution in an organic polar solvent of a polyamide acid synthesised from pyromellitic acid dianhydride and 4.4- diaminodiphenylether) sold on the market under the product name of Pire-ML (E. I. du Pont de Nemours & Co., Inc., U.S.A.).

It was found that a varnish in which a polyimide solution for use in the invention was used by itself produced `a coating film which lacked bakeability and, on an annealed copper Wire, developed cracks when the wire was wound around a wire of its own diameter.

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a convertible polyester, said convertible polyester being a phenolic-solvent-soluble condensate of substantially equivalent amounts of a dicarboxylic acid, a dihydric alcohol, and a member of the group consisting of polycarboxylic acids having at least three carboxyl groups and polyhydric alcohols having at least three hydroxyl groups, and being capable of further condensation, and a phenolic solvent, the ratio of polyester to polyirnide being from 10:1 to 1:10 by Weight.

2. A composition as claimed in claim 1 in which said convertible polyester is derived from terephthalic acid as a dicarboxylic acid component, ethylene glycol as a dihydroxy compound, and a polyhydroxy compound selected from the group consisting of glycerine as a trivalent or higher valency hydroxy compound and tris (hydroxymethyl) isocyanurate.

3. A composition as claimed in claim 1, which contains, with respect to 100 parts by weight of the resin content thereof, less than 50 parts by weight of at least one diiso- References Cited UNITED STATES PATENTS 2,982,754 5/ 1961 Sheller et al. 260--33..4 3,122,451 2/1964 lBunge 717-232 3,440,215 3/1965 Holub 260-47 3,489,696 1/1970 Miller 260-2.5

MORRIS LI-EBMAN, Primary Examiner R. ZAI'IFLEN, Assistant Examiner U.S. Cl. X.R. 260--78 TF, 857 

